Generally, a liquid crystal display has a structure in which a space between glass plates forming transparent electrodes is filled with a liquid crystal and polarizers are arranged before and after the glass plates. A polarizer used in such a liquid crystal display is manufactured in a procedure in which iodine or a dichloic dye is subjected to be adsorbed to a polyvinyl alcohol film and the film is stretched in a given direction. The polarizer thus manufactured itself absorbs light vibrating in one direction and transmits only light vibrating in the other direction therethrough to thereby produce linearly polarizing light. Therefore, an efficiency of the polarizer could not exceed 50% theoretically, which works as the greatest factor to reduce an efficiency of a liquid crystal display. As the matters worse about the absorbed light, if a liquid crystal display is operated with an increased output of a light source beyond a level, it results in inconveniences that a polarizer is broken down by heat generation due to thermal conversion of absorbed light or that a display quality is degraded under thermal influence onto liquid crystal layer in a cell.
A cholesteric liquid crystal having a circularly polarized light separating function has a selective reflection characteristic reflecting only circularly polarized light having a direction thereof coinciding with a helical rotation direction of the liquid crystal and a wavelength equal to a helical pitch length of the liquid crystal. With this selective reflection characteristic used, only a specific circularly polarizing light of natural light in a given wavelength band is transmission-separated and the other light components are reflected and recycled, thereby enabling a polarizing film with a high efficiency to be manufactured. In the context, transmitted circularly polarized light passes through a λ/4 plate and thereby converted to linearly polarizing light, and coincidence of a direction of the linearly polarized light with a transmission direction of an absorption polarizer used in a liquid crystal display enables a liquid crystal display with a high transmittance to be realized. That is, in a case where a cholesteric liquid crystal film is combined with a λ/4 plate and the combination is used as a linearly polarizer, the linearly polarizer could achieve a brightness twice as that of a conventional absorption polarizer singly used, which absorbs 50% of incident light, due to no light loss theoretically.
There has been, however, difficulty in covering all the range of visible light, since a selective reflection characteristic of a cholesteric liquid crystal is restricted to only a specific wavelength band. A selective reflection wavelength bandwidth Δλ is expressed by following formula:Δλ=2λ·(ne−no)/(ne+no)where no: ordinary light refractive index of a cholesteric liquid crystal molecule, ne: extraordinary light refractive index of the cholesteric liquid crystal molecule, and λ: central wavelength in selective reflection.
The selective reflection wavelength bandwidth Δλ depends on a molecular structure of the cholesteric liquid crystal itself. According to the above formula, if (ne−no) is larger, a selective reflection wavelength bandwidth Δλ can be broader, while (ne−no) is usually 0.3 or less. With this value being larger, other functions as a liquid crystal (such as alignment characteristic, a liquid crystal temperature or the like) becomes insufficient, causing its practical use to be difficult. Therefore, a selective reflection wavelength bandwidth Δλ has been actually on the order of 150 nm at highest. A cholesteric liquid crystal available in practical aspect has had a selective reflection wavelength bandwidth Δλ only of the order in the range of 30 to 100 nm in many cases.
A selective reflection central wavelength λ is given by the following formula:λ=(ne−no)P/2where P: helical pitch length required for one helical turn of cholesteric liquid crystal.
With a given pitch length, a selective reflection central wavelength λ depends on an average refractive index and a pitch length of a liquid crystal molecule.
Therefore, in order to cover all the range of visible light, there have been adopted methods, in one of which plural layers having respective different selective reflection central wavelengths are laminated, and in another of which a pitch length is continuously changed in the thickness direction to thereby form a positional distribution of selective reflection central wavelengths.
For example, there can be exemplified a method in which a pitch length is continuously changed in the thickness direction (for example, see a publication of JP-A No. 6-281814, a specification of JP No. 3272668 and a publication of JP-A No. 11-248943). This method is such that when a cholesteric liquid crystal composition is ultraviolet exposure-cured, exposure intensities on sides of exposure and light emission are differentiated therebetween to alter a polymerization speed therebetween, which provides a change in compositional ratio of a liquid crystal composition having a different reaction speed in the thickness direction.
The bottom line of this method lies in that exposure intensities on sides of exposure and light emission are greatly different therebetween. Therefore, in many of the examples of the prior art described above, there has been adopted a method in which an ultraviolet absorbent is mixed into a liquid crystal composition so as to cause absorption thereof in the thickness direction to thereby amplify a difference in exposure dosage according to an optical path length.
In a method disclosed in a publication of JP-A No. 6-281814, in which a pitch length is continuously altered, necessities arise for a liquid crystal thickness required for revealing the function to be on the order in the range of from 15 to 20 μm, and for more of an expensive liquid crystal in amount in addition to a problem of precise coating of a liquid crystal layer, which disables cost-up to be avoided. Moreover, an exposure time is necessary to be on the order in the range of from 1 to 60 min, which leads to a need for a long manufacturing line with an exposure line length in the range of from 10 to 600 m in order to obtain a line speed of 10 m/min. With a reduced line speed adopted, a line length can be reduced, while a lower manufacturing speed cannot be avoided.
This is because, as described in the publication of JP-A No. 6-281814, a quick change in pitch is difficult to be realized due to a theoretical issue in controlling a cholesteric pitch caused by a difference in ultraviolet exposure intensity in the thickness direction for a change in pitch length in the thickness direction and by a change in compositional ratio due to material transfer caused by a difference in polymerization speed accompanying the difference in ultraviolet intensity. Since, in the publication of JP-A No. 6-281814, pitch lengths in the short pitch side and the long pitch side are different therebetween by as large as on the order of 100 nm, a compositional ratio is necessary to change to a great extent and in order to realize it, a further necessity arises for a considerable thickness of liquid crystal, a very weak ultraviolet illumination and a long exposure time.
Since in a method disclosed in a publication of JP-A No. 11-248943, transfer of a material changing a pitch is better than an example material used in the publication of JP-A No. 6-281814, an exposure dosage of the order of 1 min enables a film to be formed. In this case as well, a necessary thickness is 15 μm, however.
While, in a specification of JP No. 3272668, a temperature condition in a first exposure is altered from that in a second exposure and a time necessary for a change in compositional ratio in the thickness direction is separately provided in a dark place, a wait time for material transfer due to a change in temperature is necessary to be in the range of from 10 to 30 min.
A liquid crystal coat thickness, even in the specification of JP No. 3272668 and the publication of JP-A No. 11-248943, is about 15 μm and in comparison of the specification and the publication described above with the publication of JP-A No. 6-281814 in which the liquid crystal coat thickness is required to be about 20 μm, it is understand that a necessity arises for a larger cholesteric liquid crystal thickness and a longer time for material transfer in order to cover all the range of visible light with a change in pitch caused by a change in compositional ratio in the thickness direction of one liquid crystal layer.
In a publication of JP-A No. 9-189811, at least three layers are necessary in order to cover all the range of visible light, and a long wavelength side is covered for betterment of a viewing angle characteristic, and the number of necessary laminated layers increases to as large as 4 to 5 in a case where a measure is taken against oblique incident light, which leads to more complexity in manufacture steps and increase in the number of steps, thereby unavoidably resulting in reduction in production yield.
With combination of such a broad band circularly polarizing plate with a retardation plate, a diffuse light source is enabled to emit collimated light. Adoption of such a collimated light source and a diffuse plate enables a construction of a viewing angle magnification system in a liquid crystal display.
For example, as shown in a specification of JP No. 2561483 and a publication of JP-No. 10-321025, by inserting a retardation plate controlled in a way such that a retardation value in a vertical direction of incidence and a retardation value in an oblique direction of incidence are specifically different from each other between polarizers, an angular distribution of transmitted light receives a restraint and in this case, if an absorption polarizer is used, light only in the vicinity of the front face is transmitted, while peripheral light are all absorbed. By using a circularly polarizing plate (a reflection polarizer), light only in the vicinity of the front face is transmitted while peripheral light is all reflected. If such an effect is adopted, emission light of a backlight can be condensed and collimated without being accompanied by absorption loss.
With combination of such a collimated backlight source and a diffuse plate less of backscattering and occurring no polarization cancellation, a viewing angle magnification system can be constructed. As described above, in a conventional method in which multiple liquid crystal layers are laminated, however, (the publication of JP-A No. 9-189811), there has arisen a problem of increased number of steps due to lamination of multiple layers, while in a method as disclosed in the publication of JP-A No. 6-281814 or the specification of JP No. 3272668 in which a liquid crystal layer is thick, there has occurred a problem of cost-up.